Online rotating viscometer and associated measurement methods

ABSTRACT

Rotating type viscosimeter having a motor ( 2 ) for driving a rotating assembly ( 3 ) extending through an aperture ( 4 ) into a liquid to be tested ( 1 ) and measuring the shear forces imparted by said liquid ( 1 ) to the rotating assembly ( 3 ). In order to prevent said liquid ( 1 ) from leaking through the aperture ( 4 ) due to a liquid pressure (p 1 ) at the level of the aperture ( 4 ), the viscosimeter comprises means for creating a counter-pressure (p 2 ) at the level of a gap between the rotating assembly ( 3 ) and the aperture ( 4 ). Thanks to this counter-pressure (p 2 ), no solid sealing means are required, thereby improving the durability and the robustness of the viscosimeter, as well as simplifying it by eliminating countermeasures otherwise needed for taking into account parasitic friction forces exerted on the rotating assembly ( 3 ) due to such solid sealing means.

FIELD OF THE INVENTION

The invention relates to a viscosimeter comprising a motor for drivingan assembly in a rotary movement, said assembly extending at leastpartially into a liquid to be tested through an aperture of a partitionfor separating the liquid from the motor.

DESCRIPTION OF PRIOR ART

Such viscosimeters are known from prior art and are often referred to asrotating viscosimeters because they either measure the shear forcesimparted by the liquid on a part of the rotating assembly in contactwith the liquid, usually a form of impeller, or on a further part inreaction to this rotation, said shear forces reflecting the viscosity ofthe liquid.

In case the liquid to be tested is contained in a container and theliquid level is able to come above the level of the aperture or in casesaid liquid flows through a pipeline, such known rotating viscosimetersoften comprise gaskets for preventing the liquid from escaping throughthe aperture. Gaskets are well known for being an important source oftrouble, particularly when they are in contact with assemblies having arelative movement with them and/or when the liquid has strong abrasiveproperties. Moreover, the frictional forces between such gaskets and therotating assembly negatively influences the accuracy of the viscositymeasurement by introducing a parasitic torque which varies a.o. withoperating conditions and time.

Means for eliminating this parasitic torque have been disclosed, forexample, by Brookfield, an American company. Brookfield uses twoconcentric shafts, an outer drive shaft and an inner sensor shaft. Thedrive shaft provides the main rotational driving force to the impellerand shields the inner sensor shaft from the frictional forces betweenthe drive shaft and an outer gasket material.

Another example of such known viscosimeters is described in patentnumber U.S. Pat. No. 5,684,247. This exemplary viscosimeter comprisesthree concentric shafts, and two gaskets, thereby making the system evenmore complicated.

Though such known viscosimeters work well in many circumstances, theyare of a complicated construction and they may not deliver thedurability and robustness required for industrial applications.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a viscosimeter with asimplified construction, with a higher durability and with a betterrobustness with regard to corrosion and/or abrasion form the liquid tobe tested.

To this end, the viscosimeter according to the invention ischaracterized in that it comprises means for creating a counter-pressureat a motor side of the partition for preventing the liquid from escapingthrough the aperture, the level of the liquid being—in operation—abovethe level of the aperture.

When the liquid to be tested is in a container and the level of theliquid comes above the level of the aperture through which the rotatingassembly extends, said liquid exerts a pressure at the level of theaperture at a liquid side of the partition, which would naturally causethe liquid to leak through the aperture. By creating a counter-pressureat the motor side of the partition, the liquid pressure iscounterbalanced, thereby stopping any leakage of the liquid. A gasket orany other form of solid seal surrounding the rotating assembly becomesthus superfluous.

The absence of a gasket greatly improves the durability and therobustness of the viscosimeter. In the absence of a gasket, theaforementioned problem of parasitic torque is also eliminated. Therotating assembly can therefore be simplified. A simple single shaftmay, for example, be used for transmitting the rotary movement from themotor to the impeller.

When the liquid to be tested is transported in a pipeline, similarproblems occur for measuring the viscosity of the liquid directly in thepipeline. The viscosimeter according to the invention solves theseproblems in a similar way, i.e. by applying a counter-pressure at themotor side for counterbalancing the liquid pressure at the level of anaperture created in the pipeline for passing through the rotatingassembly.

In many cases, the pressure exerted by the liquid at the level of theaperture is variable. This is for example the case when a tank is filledup with the liquid to be tested and conversely when the tank is emptied.Therefore, in preferred embodiments, the means for creating thecounter-pressure comprises means for regulating said counter-pressure.

SHORT DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 shows a rotating type viscosimeter according to an embodiment ofpresent invention;

FIG. 2 shows a viscosimeter according to a preferred embodiment ofpresent invention;

FIG. 3 shows a viscosimeter according to a most preferred embodiment ofpresent invention;

FIG. 4 shows another way of arranging certain components of theviscosimeter of FIG. 3;

FIG. 5 shows an example of integration of pressure generation means intothe motor housing of a viscosimeter according to the invention;

FIG. 6 a and 6 b show two exemplary arrangements of the motor withregard to the partition in an embodiment according to the invention, and

FIG. 7 shows an embodiment of a viscosimeter according to the inventionwith a safety membrane.

The figures are not drawn to scale. Generally, identical components aredenoted by the same reference numerals in the figures.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows schematically a rotating viscosimeter according to theinvention and set up for the measurement of the viscosity of a liquid tobe tested (1). The viscosimeter comprises a motor (2) for driving arotating assembly (3) extending into the liquid to be tested (1) throughan aperture (4) of a partition (5) separating the liquid to be tested(1) from the motor (2).

The partition (5) defines thus a separation between a liquid side (L)and a motor side (M), the motor side (M) being the side where one wantsto prevent the liquid to be tested (1) to penetrate. At the level of theaperture (4), the liquid to be tested (1) exerts a pressure (p1) on theliquid side (L) which would naturally cause the liquid to be tested (1)to leak through the aperture (4) if there were no means for preventingsuch leakage. A viscosimeter according to the invention comprises meansfor creating a counter-pressure (p2) at the motor side (M) and at thelevel of the aperture (4), such that p2>p1, thereby preventing theliquid to be tested (1) from leaking through the aperture (4). In ordernot to influence the measurement, the power for creating saidcounter-pressure (p2) does preferably not originate from the motor (2),but rather from an independent power source. As long as saidcounter-pressure (p2) is present, which is the case during normaloperation of the viscosimeter, there is no contact between the rotatingassembly (3) and the partition (5) or any other part attached to thepartition (5), neither directly nor indirectly such as via a solidsealing material like a gasket for example. In normal operation, therotating assembly (3) thus rotates freely into the liquid to be tested(1) without friction forces due to solid sealing means.

An exemplary way for creating such counter-pressure is illustrated onFIG. 2. Here, the viscosimeter comprises a motor housing (10) whichencloses the motor (2) hermetically with regard to the partition (5),except for the aperture (4). The motor housing (10) is pressurised byfeeding it with compressed air at a supply pressure (ps), such that ps³p2. A pressure valve (20) can be used for setting the desiredcounter-pressure (p2). A check-valve (21) may also be used forpreventing back-pressure from the motor housing (10) to the compressedair supply system. Such check-valve (21) also serves for maintaining thecounter-pressure (p2) in the motor housing (10) in case the supply ofcompressed air would fail.

Venting of the motor housing (10) usually occurs through the aperture(4), though an additional safety valve can be installed on the motorhousing (10).

As with most control systems, a closed loop regulation may be requiredin order to improve the accuracy of the control and/or to improve it'sresponse time. Therefore, in a preferred embodiment, the viscosimeteraccording to the invention comprises means for regulating thecounter-pressure (p2), as illustrated on FIG. 3. Said means forregulating the counter-pressure comprise:

-   -   a liquid pressure sensor (25) for measuring the pressure (p1)        exerted by the liquid to be tested (1) on the liquid side (L) at        the level of the aperture (4),    -   an air pressure sensor (26) for measuring the counter-pressure        (p2) at the motor (2) side (M) at the level of the aperture (4),    -   means (40) for comparing p1 and p2, and for actuating the        pressure valve (20) depending on the results of the comparison        and in order to achieve the desired counter-pressure (p2).

There are various ways for arranging those means for regulating thecounter-pressure. To take advantage of the pressurisation, it may forexample be advantageous to arrange said means as well as the pressurevalve (20) and the check-valve (21) into the motor housing (10), asillustrated on FIG. 4.

For the same reasons, and/or for further reasons, such as for reasons ofintegration, it may be advantageous to further arrange means forgenerating compressed air (50) into the motor housing (10), asillustrated on FIG. 5.

There are also various ways for arranging the motor (2) with regard tothe partition (5). In one embodiment, the motor (2) is so arranged thatthere would be no way for the pressurised air to access the aperture(4). An exemplary arrangement is shown on FIG. 6 a, where the motor (2)has a lateral side (2 a) which is flattened against the partition (5).With such an arrangement, additional air ducts (60, 61) are arranged inthe partition (5) in order to bring the pressurised air to the level ofthe aperture (4) for providing the counter-pressure (p2) to the liquid(1). In other embodiments, the lateral side (2 a) of the motor (2) iskept away from the partition (5) by appropriate distance pieces (65,66), as illustrated on FIG. 6 b. In this case, no additional air ductsare necessary: the pressurized air flows around the distance pieces (65,66) to deliver the counter-pressure (p2) at the level of the aperture(4).

In embodiments, a gap between the rotating assembly (3) and the aperture(4) is greater than or equal to 0.01 mm and less than or equal to 5mm.Preferably said gap is greater than or equal to 0.1 mm and less than orequal to 1 mm. Such a gap offers a good compromise between themanufacturing accuracy and the power needed for generating andcontrolling the counter-pressure (p2). Evidently, said gap is to beunderstood as being the smallest distance between the aperture throughwhich the rotating assembly extends and the rotating assembly. In caseof an assembly as illustrated in FIG. 6 a and 6 b for example, the gapis indicated by the letter G.

The viscosimeter according to the invention may in embodiments alsocomprise automatic sealing means for preventing the liquid (1) fromescaping through the aperture (4) following to an insufficientcounter-pressure (p2), i.e. when p2 becomes smaller than p1. Thecounter-pressure (p2) may for example become insufficient because of afailure of the means for creating the counter-pressure, or simplybecause said means are voluntary deactivated for maintenance reasons.

An example of such an automatic sealing means is shown in FIG. 7. Theautomatic sealing means comprises a safety membrane (70) which ishermetically attached to the rotating assembly (3) and whose apparentsurface overlaps the aperture (4). The safety membrane (70) is arrangedin such a way that, under normal operating conditions, it is pushed awayfrom the partition (5) by the compressed air due to the counter-pressure(p2). When the counter-pressure (p2) falls below the liquid pressure(p1) at the level of the aperture (4), the liquid (1) will automaticallypush the safety membrane (70) against the partition (5), therebypreventing the liquid (1) from escaping through the aperture (4). It isto be noted that since the safety membrane (70) is pushed away from thepartition (5) under normal operating conditions, said membrane (70) doesnot generate parasitic friction forces.

In a preferred embodiment, the viscosimeter according to the inventionis used for the online measurement of the viscosity of a materialcomprising a polymer in the course of polymerisation. The inventor hasoptimised the components for such an application, leading to thefollowing characteristics:

-   -   the motor (2) is a DC motor (2) with a nominal power of 90 W,    -   the volume of air contained in the motor housing (10) is around        30 cl (centiliters),    -   the counter-pressure (p2) is set at 140 kPa relative for a        liquid level which is 6 m above the level of the aperture (4).

It is to be noted that other gasses than air can be used for providingthe counter-pressure (p2). In case a reaction of the gas with the liquidto be tested (1) is undesirable or unacceptable, an inert gas such asnitrogen can be used.

So far, the term viscosity has been used, which is typically a propertyof fluids. It is to be understood that the invention also relates toapparatus for the measurement of similar properties of multi-phasematerials, yet using a similar measurement principle, such as forexample apparatus measuring the shear forces imparted by a slurry on arotating part for determining the consistency of said slurry.

Evidently, the viscosimeter according to the invention may also be usedin cases where the liquid level is not able to reach the level of theaperture (4), for example for measuring the viscosity of a liquidcontained in a tank through an aperture (4) which is above the levelwhich the liquid is able to reach.

In short the invention may be described as follows: Rotating typeviscosimeter having a motor (2) for driving a rotating assembly (3)extending through an aperture (4) into a liquid to be tested (1) andmeasuring the shear forces imparted by said liquid (1) to the rotatingassembly (3). In order to prevent said liquid (1) from leaking throughthe aperture (4) due to a liquid pressure (p1) at the level of theaperture (4), the viscosimeter comprises means for creating acounter-pressure (p2) at the level of a gap between the rotatingassembly (3) and the aperture (4). Thanks to this counter-pressure (p2),no solid sealing means are required, thereby improving the durabilityand the robustness of the viscosimeter, as well as simplifying it byeliminating countermeasures otherwise needed for taking into accountparasitic friction forces exerted on the rotating assembly (3) due tosuch solid sealing means.

1. Viscosimeter comprising a motor (2) for driving an assembly (3) in arotary movement, said assembly (3) extending at least partially into aliquid to be tested (1) through an aperture (4) of a partition (5) forseparating the liquid (1) from the motor (2), characterized in that theviscosimeter comprises means for creating a counter-pressure (p2) at amotor side (M) of the partition (5) for preventing the liquid (1) fromescaping through the aperture (4), the level of the liquid (1) being—inoperation—above the aperture (4).
 2. Viscosimeter according to claim 1,further comprising means (40) for regulating the counter-pressure (p2).3. Viscosimeter according to claim 2, characterized in that a gapbetween the rotating assembly (3) and the aperture (4) at the level ofthe aperture (4) is greater than or equal to 0.01 mm and less than orequal to 5 mm.
 4. Viscosimeter according to claim 3, characterized inthat the gap is greater than or equal to 0.1 mm and less than or equalto 1 mm.
 5. Viscosimeter according to claim 4, further comprisingautomatic sealing means for preventing the liquid (1) from escapingthrough the aperture (4) in case the means for creating thecounter-pressure (p2) at the motor side (M) of the partition (5) fail toprevent the liquid (1) from escaping through the aperture (4). 6.Viscosimeter according to claim 5, characterized in that the automaticsealing means comprise a safety membrane (70) hermetically attached tothe rotating assembly (3) and in that the apparent surface of saidsafety membrane (70) overlaps the aperture (4).
 7. Viscosimeteraccording to claim 6, characterized in that a medium for exerting thecounter-pressure (p2) is ambient air.
 8. Viscosimeter according to claim6, characterized in that a medium for exerting the counter-pressure (p2)is an inert gas.
 9. Viscosimeter according to claim 1, characterized inthat a gap between the rotating assembly (3) and the aperture (4) at thelevel of the aperture (4) is greater than or equal to 0.01 mm and lessthan or equal to 5 mm.
 10. Viscosimeter according to claim 9characterized in that the gap is greater than or equal to 0.1 mm andless than or equal to 1 mm.
 11. Viscosimeter according to claim 1,further comprising automatic sealing means for preventing the liquid (1)from escaping through the aperture (4) in case the means for creatingthe counter-pressure (p2) at the motor side (M) of the partition (5)fail to prevent the liquid (1) from escaping through the aperture (4).12. Viscosimeter according to claim 11, characterized in that theautomatic sealing means comprise a safety membrane (70) hermeticallyattached to the rotating assembly (3) and in that the apparent surfaceof said safety membrane (70) overlaps the aperture (4).
 13. Viscosimeteraccording to claim 1, characterized in that a medium for exerting thecounter-pressure (p2) is ambient air.
 14. Viscosimeter according toclaim 3, characterized in that a medium for exerting thecounter-pressure (p2) is ambient air.
 15. Viscosimeter according toclaim 4, characterized in that a medium for exerting thecounter-pressure (p2) is ambient air.
 16. Viscosimeter according toclaim 5, characterized in that a medium for exerting thecounter-pressure (p2) is ambient air.
 17. Viscosimeter according toclaim 1, characterized in that a medium for exerting thecounter-pressure (p2) is an inert gas.
 18. Viscosimeter according toclaim 3, characterized in that a medium for exerting thecounter-pressure (p2) is an inert gas.
 19. Method for measuring theviscosity of a liquid to be tested (1), comprising the steps of: havinga motor (2) driving an assembly (3) in a rotary movement, said assembly(3) extending at least partially into the liquid to be tested (1)through an aperture (4) of a partition (5) for separating the liquid (1)from the motor (2), the level of the liquid (1) being—in operation—abovethe aperture (4), creating a counter-pressure (p2) at a motor side (M)of the partition (5) for preventing the liquid (1) from escaping throughthe aperture (4), and measuring the shear forces imparted by the liquid(1) on a part of the rotating assembly (3) in contact with the liquid(1).
 20. Method for measuring the viscosity of a liquid according toclaim 19, further comprising the step of regulating the counter-pressure(p2).